Tidal stream generators generally use vanes or blades which rotate about a horizontal axis. GB 2 348 250 A describes a tidal stream generator which includes a support column mounted on a river or sea bed. The generator includes a turbine unit with blades which rotate about a horizontal axis. The turbine unit can be moved up and down the column by means of a cylindrical tank held inside the column. Water can be removed from the tank by displacing it with compressed air to lift the tank by buoyancy to a position in which the tank projects above the surface of the water. The turbine unit is fixed to the tank, so that when the tank is in its lifted position the turbine unit is at least partially above the surface of the water. The support column has a longitudinal slot to allow the turbine unit, which is fixed to the tank held inside the column, to rise and fall with the tank. The support column is weakened by the provision of the longitudinal slot. The turbine unit can face in one direction only, so that the generator can generate maximum power only when the tidal flow is parallel to the axis of rotation of the turbine unit.
It is known to provide a tidal stream generator with vanes which rotate about a vertical axis. WO 2008/050149 A1 discloses a tidal stream generator comprising a moored floating pontoon with a horizontal duct extending therethrough. A turbine is mounted in the duct on a vertical axis, and vertical deflector vanes in the duct deflect the water flow onto the vanes of the turbine. The turbine can only be positioned at a level just below the surface of the water. The pontoon is moored in a fixed position and can only generate maximum power when the tidal flow is parallel to longitudinal axis of the duct.
A further problem with prior art tidal stream generators is that they are optimised to work at a particular tidal flow rate. A rotary electric generator is designed to provide its maximum efficiency at a particular rotary speed. A water turbine in a tidal power generator apparatus is rotationally coupled to the electric generator, such that as the tidal flow rate increases, the water speed of the water turbine increases, and the rotary speed of the electric generator increases. At tidal flow rates higher or lower than the design tidal rate, the generator does not operate at its maximum efficiency.
It is an object of the present invention to overcome one or more of the above problems.